Sealing: Virtually all radon in Wisconsin comes from the soil beneath houses. Radon at 300 to 1,000 pCi/L or more is in the gasses in the soil under basement floors everywhere. Gaps and openings to soil through basement floors and walls should be sealed with gas-tight materials. The caulk type with the best adhesion to concrete is polyurethane (not silicone). However, experience by researchers has shown that sealing cracks and openings in basements will result in reductions of radon by more than 50% in only about 20% of the houses in which it is applied. Since it can be inexpensive and is part of the next method, it is worth a try, but one shouldn't expect it will necessarily have a major effect. It might reduce radon levels significantly if the area of openings that are sealed adds up to several square inches. Hairline cracks are not worth sealing.

Soil Depressurization is usually highly effective, reducing radon to below 2 pCi/L. Air is withdrawn from beneath the basement floor with a continuously-running fan in a 3 or 4-inch diameter pipe which exhausts at roof level. This reduces the air pressure below the floor, so air in the basement flows down to the depressurized zone, through small cracks and openings that could not be sealed, instead of soil gases containing radon flowing up into the basement through those openings. The State of Wisconsin has distributed to all public libraries in Wisconsin copies of an eleven-minute videotape, Radon Reduction: Sub-Slab Depressurization, showing a system installed and how it works.

Cost: A proficiency-listed contractor can install a system for around $1,200, although it can range from $800 to $2,000. Sub-slab depressurization is not a do-it-yourself project unless you have considerable contractor skills.

Radon Mitigation System Standards

To get the radon as low as reasonably possible, maximize durability, and minimize operating costs for homeowners, regulating states have adopted standards for mitigation systems. The US EPA document, Radon Mitigation Standards, (PDF, 1 MB) has been the model. In it, some of the most important features for good mitigation systems are described in Sections14.2 (vent pipe) (PDF, 212 KB), 14.3 (fan installation) (PDF, 141 KB), and 14.5 (sealing) (PDF, 226 KB).

What does a radon mitigation system look like?

There are several online resources that will give you an idea of what is typically done during the installation of a radon mitigation system:

"This Old House" featured a short segment that gives an overview on how to install a mitigation system.